Cell lines comprising sour-taste receptors

ABSTRACT

The present invention relates to sour taste receptors and compositions and methods thereof. In particular, the present invention provides assays and methods of screening for ligands specific for sour taste receptors. Additionally, the present invention provides methods for screening for accessory proteins and mutations, polymorphisms and other potential sour taste receptor protein mutations that are associated with disease states, and therapeutic agents, ligands, and modulators of such proteins. The present invention also provides compositions and methods for modulating sour taste receptors in vitro and in vivo.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is a Continuation of pending U.S. patentapplication Ser. No. 12/632,299, filed Dec. 7, 2009, which will issue onAug. 23, 2011 as U.S. Pat. No. 8,003,384, which is a Continuation ofpending U.S. patent application Ser. No. 11/825,941, filed Jul. 10,2007, now issued as U.S. Pat. No. 7,629,134, which claims priority toexpired U.S. Provisional Patent Application No. 60/819,675 filed Jul.10, 2006, all of which are herein incorporated by reference in theirentireties.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under Grant No. 5 ROIDC005782 awarded by the National Institutes of Health. The governmenthas certain rights in the invention.

FIELD OF THE INVENTION

The present invention relates to sour taste receptors and compositionsand methods thereof. In particular, the present invention providesassays and methods of screening for ligands specific for sour tastereceptors. Additionally, the present invention provides methods forscreening for accessory proteins and mutations, polymorphisms and otherpotential sour taste receptor protein mutations that are associated withdisease states, and therapeutic agents, ligands, and modulators of suchproteins. The present invention also provides compositions and methodsfor modulating sour taste receptors in vitro and in vivo.

BACKGROUND OF THE INVENTION

Flavor is a complex mixture of sensory input composed of taste(gustation), smell (olfaction) and the tactile sensation of food as itis being munched, a characteristic that food scientists often term“mouthfeel.” Although people may use the word “taste” to mean “flavor,”in the strict sense it is applicable only to the sensations arising fromspecialized taste cells in the mouth. Scientists generally describehuman taste perception in terms of four qualities: saltiness, sourness,sweetness and bitterness. A fifth taste exists as umami, the sensationelicited by glutamate, one of the 20 amino acids that make up theproteins in meat, fish and legumes. Glutamate also serves as a flavorenhancer in the form of the additive monosodium glutamate (MSG).

Animals use taste systems to evaluate the nutritious value, toxicity,sodium content, and acidity of the food they ingest. In vertebrates,taste reception occurs at the top of the taste cells that form tastebuds, and each taste bud has an onion-like shape. There are four majortaste areas where taste buds are concentrated; on the tongue at thecircumvallate papilla, foliate papilla, and fungiform papilla, and thepalate (top of the mouth). Circumvallate papillae, found at the veryback of the tongue, contain hundreds to thousands of taste buds. Bycontrast, foliate papillae, localized to the posterior lateral edge ofthe tongue, contain dozens to hundreds of taste buds. Further, fungiformpapillae, located at the front of the tongue, contain only a single or afew taste buds. Each taste bud, depending on the species, contains50-150 cells, including precursor cells, support cells, and tastereceptor cells (Lindemann et al., 1996, Physiol. Rev. 76:718-66).Receptor cells are innervated at their base by afferent nerve endingsthat transmit information to the taste centers of the cortex throughsynapses in the brain stem and thalamus. Elucidating the mechanisms oftaste cell signaling and information processing is important tounderstanding the function, regulation, and perception of the sense oftaste.

Much progress has been made in unraveling molecular mechanisms ofbitter, sweet and umami taste in recent years (Margolskee, 2002, J.Biol. Chem. 277:1-4; Montmayeur and Matsunami, 2002, Curr. Opin.Neurobiol. 12:366-371; Scott, 2004, Curr. Opin. Neurobiol. 14:423-427).However, the molecular basis of sour taste sensation is the most poorlyunderstood of the five basic modalities.

A whole industry exists around trying to disguise or mask unpleasanttastes. In 1879, Ira Remsen noticed that a derivative of coal tar tastedsweet. His finding led to the development of saccharin, an artificialsweetener today known as Sweet-n-Low Brand® sweetener. Today, many moreartificial sweeteners with varying chemical structures are availableincluding Sunett® (acesulfame potassium), NutraSweet® or Equal®(aspartame), Splenda® (sucralose), and Sugaree® (D-Tagatose). However,some of these artificial sweeteners, such as saccharin and aspartame,have been linked with cancer and other medical problems. Natural plantcompounds have also been found to mask unpleasant tastes. Miraculin, aprotein found in the pulp of the fruit of the miracle berry, anevergreen shrub native to West Africa, has been described as a“sweet-inducing” protein, and is suggested to bind to sweet tastereceptors in the mouth when sour substances are present, the resultbeing a strong sweet taste. Miraculin itself has no distinct taste, butthe human tongue when exposed to the protein perceives ordinarily sourfoods as sweet. Other plant proteins which are being studied as naturalsweeteners include, stevia, curculin, mabinlin, monellin, pentadin,brazzein, and thaumatin (Faus, 2000, Appl. Microbiol. Biotechnol.53:145-151; Kohmura et al., 2002, Pure Appl. Chem. 74:1235-1242).Contrasted to those individuals who prefer sweet tasting products, thereare an equal number who seek out the taste of sour, as evidenced by themyriad of sour candy options available for consumption.

Sweeteners, either artificial or natural, find useful application, forexample, as sugar substitutes in the weight loss industry, as sugaralternatives for people suffering from diabetes and other diseases wheresugar intake is restricted, as additives to foods and beverages, and inthe pharmaceutical industry to make medicaments palatable. Clinically,taste disorders are prevalent in patients undergoing chemotherapy andoften have a negative impact on the quality of life and nutrition forthose patients. Radiation treatment can also damage taste receptors,giving food a metallic taste. Those patients suffering from tastedistortion may avoid foods with high nutritional value, such as freshfruits and vegetables, thereby further depressing their immunefunctions. A better understanding of the complex and oftenmultifactorial etiology of taste dysfunction would enable the clinicianto institute measures to minimize the impact of these disturbingchanges. What is needed is a better understanding of sour taste receptorsensation. What is further needed is a better understanding of sourtaste receptor function. Additionally, what is needed are methods andassays to screen for, and to use, ligands that can either inhibit orupregulate sour taste receptor.

SUMMARY OF THE INVENTION

The present invention relates to sour taste receptors and compositionsand methods thereof. In particular, the present invention providesassays and methods of screening for ligands specific for sour tastereceptors. Additionally, the present invention provides methods forscreening for accessory proteins and mutations, polymorphisms and otherpotential sour taste receptor protein mutations that are associated withdisease states, and therapeutic agents, ligands, and modulators of suchproteins. The present invention also provides compositions and methodsfor modulating sour taste receptors in vitro and in vivo.

The transient receptor potential (TRP) ion channel subunit genes werefirst defined in the Drosophila visual system, where TRP deficient flieswere blinded by intense light as a result of calcium dependentadaptation disruption (Clapham et al., 2002, IUPHAR Compendium, TRPChannels). Since then, TRP ion channels have been implicated in varioussensory systems, including vision, smell, pheromone, hearing, touch,osmolarity, thermosensation, and sweet, bitter and umami taste, indiverse animal species ranging from mammals and fish to fruit flies andnematodes (Clapham, 2003, Nature 426:517-524; Montell, 2005, Sci. STKE2005:re3). Some TRP channels such as vanilloid receptor, TRPV 1,function as receptors for stimuli (high temperature and capsaicin) bythemselves, whereas other TRP channels, such as TRPM5, are downstreameffectors of G protein coupled sensory receptors.

Two TRP channel family members, PKD1L3 and PKD2L1, are co-expressed in asubset of taste receptor cells in specific taste areas. Cells expressingthese molecules are different from bitter, sweet or umami sensing cells.The PKD2L1 proteins are accumulated at the taste pore region, wheretaste chemicals are detected. Finally, PKD1L3 and PKD2L1 are activatedby sour chemicals when co-expressed in heterologous cells. Therefore,PKD1 L3 and PKD2L1 heteromers function as sour taste receptors.

In one embodiment, the present invention relates to a method foridentifying a sour taste receptor ligand, comprising providing a samplecomprising a sour taste receptor, and a test compound, exposing saidtest compound to said sample and measuring the activity of said sourtaste receptor in said sample in response to said test compound. In someembodiments, said sample is a cell line expressing PKD1L3 and PKD2L1. Insome embodiments, said cell line is a 293T cell line. In someembodiments, said cell line is derived from a 293T cell line, such as aHana3A cell line or a 44 cell line. In some embodiments, said PKD1L3 andPKD2L1 are either human or murine. In some embodiments, said testcompound is from a list consisting of a naturally occurring molecule, asynthetically derived molecule, or a recombinantly derived molecule.

In one embodiment, the method for identifying a sour taste receptorligand further comprising a reporting agent. In some embodiments, themethod for identifying a sour taste receptor ligand further comprisesthe step of detecting the presence or absence of a sour taste receptorligand based upon said reporting agent activity. In some embodiments,said reporting agent is a fluorophore, and said fluorophore is from agroup consisting of fluo-4 and fura-red.

In one embodiment, the present invention is a cell that expresses aheterologous sour taste receptor. In some embodiments, said cell lineexpresses murine or human PKD1L3 and PKD2L1, or combinations thereof. Insome embodiments, said cell is a human embryonic kidney 293T cell line.

In some embodiments, the sour taste receptor is modulated, in vivo or invitro, by the introduction of a modulator (e.g., ligand, chemical,compound, or agent) to a sample or subject such that the sour tastesensation is inhibited or decreased. In other embodiments, the modulatoracts to upregulate or increase sour taste sensation. In someembodiments, the modulator is added or applied to a food product (e.g.,vegetable, fruit, meat, candy, oils, etc.). In other embodiments, aninhibitor of the sour taste sensation is added or applied as part of apharmaceutical medicament (e.g., pillules, powders, elixirs, etc.).

DESCRIPTION OF THE FIGURES

FIG. 1 shows the localization of PKD1L3 and PKD2L1 on the mouse tongue.

FIG. 2 shows a potential mechanism for cell surface formation of thePKD1 L3 and PKD2L1 heteromer.

DEFINITIONS

The term “test compound” refers to any chemical entity, pharmaceutical,drug, and the like that can be used to treat or prevent a condition,disease, illness, sickness, or disorder of bodily function, or otherwisealter the physiological or cellular status of a sample. Test compoundscomprise both known and potential therapeutic compounds. A test compoundcan be determined to be by screening using the screening methods of thepresent invention.

The term “sample” as used herein is used in its broadest sense. A samplesuspected of containing a protein may comprise a cell, a portion of atissue, an extract containing one or more proteins and the like.

As used herein, the term “reporter gene” refers to a gene encoding aprotein that may be assayed. Examples of reporter genes include, but arenot limited to, luciferase (See, e.g., deWet et al., Mol. Cell. Biol.7:725 [1987] and U.S. Pat Nos. 6,074,859; 5,976,796; 5,674,713; and5,618,682; all of which are incorporated herein by reference), greenfluorescent protein (e.g., GenBank Accession Number U43284),chloramphenicol acetyltransferase, β-galactosidase, alkalinephosphatase, horse radish peroxidase, and fluorophores such as fluo-4and fura-red.

The term “siRNAs” refers to short interfering RNAs. Methods for the useof siRNAs are described in U.S. Patent App. No. 20030148519/A1 (hereinincorporated by reference in its entirety). In some embodiments, siRNAscomprise a duplex, or double-stranded region, of about 18-25 nucleotideslong; often siRNAs contain from about two to four unpaired nucleotidesat the 3′ end of each strand. At least one strand of the duplex ordouble-stranded region of a siRNA is substantially homologous to orsubstantially complementary to a target RNA molecule. The strandcomplementary to a target RNA molecule is the “antisense strand;” thestrand homologous to the target RNA molecule is the “sense strand,” andis also complementary to the siRNA antisense strand. siRNAs may alsocontain additional sequences; non-limiting examples of such sequencesinclude linking sequences, or loops, as well as stem and other foldedstructures. siRNAs appear to function as key intermediaries intriggering RNA interference in invertebrates and in vertebrates, and intriggering sequence-specific RNA degradation during posttranscriptionalgene silencing in plants.

The term “RNA interference” or “RNAi” refers to the silencing ordecreasing of gene expression by siRNAs. It is the process ofsequence-specific, post-transcriptional gene silencing in animals andplants, initiated by siRNA that is homologous in its duplex region tothe sequence of the silenced gene. The gene may be endogenous orexogenous to the organism, present integrated into a chromosome orpresent in a transfection vector that is not integrated into the genome.The expression of the gene is either completely or partially inhibited.RNAi may also be considered to inhibit the function of a target RNA; thefunction of the target RNA may be complete or partial.

The term “fragment” as used herein refers to a polypeptide that has anamino-terminal and/or carboxy-terminal deletion as compared to thenative protein, but where the remaining amino acid sequence is identicalto the corresponding positions in the amino acid sequence deduced from afull-length cDNA sequence. Fragments typically are at least 4 aminoacids long, preferably at least 20 amino acids long, usually at least 50amino acids long or longer, and span the portion of the polypeptiderequired for intermolecular binding of the compositions (claimed in thepresent invention) with its various ligands and/or substrates.

As used herein, the term “genetic variation information” or “geneticvariant information” refers to the presence or absence of one or morevariant nucleic acid sequences (e.g., polymorphism or mutations) in agiven allele of a particular gene (e.g., a PKD1L3 or PKD2L1 gene of thepresent invention).

The term “naturally-occurring” as used herein as applied to an objectrefers to the fact that an object can be found in nature. For example, apolypeptide or polynucleotide sequence that is present in an organism(including viruses) that can be isolated from a source in nature andwhich has not been intentionally modified by man in the laboratory isnaturally-occurring.

The term “recombinant protein” or “recombinant polypeptide” as usedherein refers to a protein molecule that is expressed from a recombinantDNA molecule.

The term “native protein” as used herein, is used to indicate a proteinthat does not contain amino acid residues encoded by vector sequences;that is the native protein contains only those amino acids found in theprotein as it occurs in nature. A native protein may be produced byrecombinant means or may be isolated from a naturally occurring source.

As used herein, the term “vector” is used in reference to nucleic acidmolecules that transfer DNA segment(s) from one cell to another. Theterm “vehicle” is sometimes used interchangeably with “vector.”

The term “expression vector” as used herein refers to a recombinant DNAmolecule containing a desired coding sequence and appropriate nucleicacid sequences necessary for the expression of the operably linkedcoding sequence in a particular host organism. Nucleic acid sequencesnecessary for expression in prokaryotes usually include a promoter, anoperator (optional), and a ribosome binding site, often along with othersequences. Eukaryotic cells are known to utilize promoters, enhancers,and termination and polyadenylation signals.

As used herein, the term “host cell” refers to any eukaryotic orprokaryotic cell (e.g., bacterial cells such as E. coli, yeast cells,mammalian cells, avian cells, amphibian cells, plant cells, fish cells,and insect cells), whether located in vitro or in vivo. For example,host cells may be located in a transgenic animal.

The term “transfection” as used herein refers to the introduction offoreign DNA into eukaryotic cells. Transfection may be accomplished by avariety of means known to the art including calcium phosphate-DNAco-precipitation, DEAE-dextran-mediated transfection, polybrene-mediatedtransfection, electroporation, microinjection, liposome fusion,lipofection, protoplast fusion, retroviral infection, and biolistics.

The term “stable transfection” or “stably transfected” refers to theintroduction and integration of foreign DNA into the genome of thetransfected cell. The term “stable transfectant” refers to a cell thathas stably integrated foreign DNA into the genomic DNA.

The term “transient transfection” or “transiently transfected” refers tothe introduction of foreign DNA into a cell where the foreign DNA failsto integrate into the genome of the transfected cell. The foreign DNApersists in the nucleus of the transfected cell for several days. Duringthis time the foreign DNA is subject to the regulatory controls thatgovern the expression of endogenous genes in the chromosomes. The term“transient transfectant” refers to cells that have taken up foreign DNAbut have failed to integrate this DNA.

DETAILED DESCRIPTION OF THE INVENTION

Bitter, sweet and umami stimuli are detected by G protein coupledreceptors. Bitter chemicals are detected by around 30 T2R receptorfamily members (Adler et al., 2000, Cell 100:693-702; Chandrashekar etal., 2000, Cell 100:703-711; Matsunami et al., 2000, Nature404:601-604). Sweet and umami compounds are detected by differentcombinations of T1R family members. Sugars and sweeteners are detectedby T1R2+T1R3 heteromers, whereas umami tasting 1-amino acids aredetected by T1R1+T1R3 heteromers (Damak et al., 2003, Science301:850-853; Kitagawa et al., 2001, Biochem. Biophys. Res. Comm.283:236-242; Li et al., 2002, Proc. Natl. Acad. Sci. 99:4692-4693; Maxet al., 2001, Nat. Genet. 28:58-63; Montmayeur et al., 2001, Nat.Neurosci 4:492-498; Nelson et al., 2002, Nature 416:199-202; Nelson etal., 2001, Cell 106:381-390; Zhao et al., 2003, Cell 115:255-266).Different sets of taste cells express T2R5, T1R2+T1R3, or T1R1+T1R3.Moreover, an animal's preference toward chemicals can be manipulated bymisexpressing foreign receptors in different subsets of taste cells. Forexample, when the artificial RASSL receptor was expressed in T1R2positive sweet sensing cells, mice were attracted to water containingspiradonine, an agonist for the RASSL receptor, whereas when the samereceptor was expressed in T2R expressing bitter sensing cells, theanimals avoid spiradoline (Mueller et al., 2005, Nature 434:225-229;Zhao et al., 2003). Thus, taste cells are likely to be “labeled” asbitter, sweet, or umami sensing cells. Nevertheless, both T1R5 and T2R5express common signal transduction molecules, including PLCb2 and TRPM5,and IP3R-3 (Clapp et al., 2001, Neurosci. 2:6; Miyoshi et al., 2001,Chem. Senses 26:259-265; Perez et al., 2002, Nat. Neurosci. 5:1169-1176;Zhang et al., 2003, Cell 112:293-301).

In contrast to sweet, bitter and umami sensations, molecular mechanismsof sensing sour and salty taste are poorly understood and evenconfusing, although a number of candidate receptors and transductionmechanisms have been proposed (Miyamoto et al., 2000, Prog. Neurobiol.62:135-157). For example, acid-sensing ion channel-2 (ASIC2) is proposedto function as a sour receptor in the rat (Ugawa et al, 2003, J.Neurosci. 23:3616-3622; Ugawa et al., 1998, Nature 395:555-556).However, it is not expressed in mouse taste cells and not required foracid sensation (Richter et al., 2004, J. Neurosci. 24:4088-4091). HCN1and HCN4, members of hyperpolarization-activated cyclic nucleotide gatedchannels (HCNs) are also candidate sour receptor channels (Stevens etal., 2001, Nature 413:631-635). However, calcium imaging experimentsusing taste bud slices did not support this possibility, as Cs⁺, aninhibitor of HCN channels, did not block Ca²⁺ response of taste cells tosour stimuli (Richter et al., 2003, J. Physiol. 547:475-483). Moreover,unlike bitter, sweet and umami taste receptors, SICS2, HCN1 and JCN4 areall widely expressed in the nervous system (Lingueglia et al., 1997, J.Biol. Chem. 272:29778-29783; Ludwig et al., 1998, Nature 393:587-591;Moosmang et al., 1999, Biol. Chem. 380:975-980).

Among TRP channel families, member of the PKD family (polycystic kidneydisease, also called TRPP or polycystins) have unique properties (Delmaset al., 2004, Biochem. Biophys. Res. Commun. 322:1374-1383; Nauli andZhou, 2004, Bioessays 26:844-856). Their founding members, PKD1 andPKD2, were identified as autosomal dominant polycystic kidney diseasegenes. PKD1 is a large protein with a long N-terminal extracellulardomain followed by 11 transmembrane domains. PKD1 may not formfunctional ion channels, while PKD2 which has 6 transmembrane domainssimilar to other TRP members, can function as a non-selective cationchannel. Importantly, PKD1 and PKD2 heteromer formation using theirintracellular C-terminal regions is required to become a functionalreceptor/channel (Hanaoka et al., 2000, Nature 408:990-994). Theheteromer of PKD1 and PKD2 are thought to sense mechanical flow,osmolarity and unknown extracellular ligand(s). In C. elegans, a PKD1homolog, Lov-1, and a PKD2 homolog are expressed in male specificsensory neurons, localized at the chemosensory cilia, and are requiredfor male mating behavior thereby suggesting their function as sensoryreceptors (Barr et al., 2001, Curr. Biol. 11:1341-1346; Barr andSternberg, 1999, Nature 401:386-389). There are four additionalPKD1-like and two additional PKD2-like genes found in the mouse or humangenome (Chen et al., 1999, Nature 401:383-386; Guo et al., 2000,Genomics 241-251; Hughes et al., 1999, Hum. Mol. Genet. 8:543-549; Li etal., 2003, Genomics 81:596-608; Nomura et al., 1998, J. Biol. Chem.273:25967-25973; Yuasa et al., 2002, Genomics 79:376-386), however thebiological functions of these PKD related molecules are poorlyunderstood.

Characterization of molecular identities that receive taste chemicals isneeded to understand the molecular mechanisms underlying tastesensation. Two TRP channel members, PKD1L3 (Genbank Accession Nos.AY164486 (murine, nucleic acid, SEQ ID NO:1), AAO32799 (murine, aminoacid, SEQ ID NO:2), AY164485 (human, nucleic acid, SEQ ID NO:3) andAAO032798 (human, amino acid, SEQ ID NO:4), incorporated herein byreference in their entireties) and PKD2L1 (Genbank Accession Nos.NM_(—)181422 (murine, nucleic acid, SEQ ID NO:5), NP_(—)852087 (murine,amino acid, SEQ ID NO:6), NM_(—)016112 (human, nucleic acid, SEQ IDNO:7) and NP_(—)057196 (human, amino acid, SEQ ID NO:8), incorporatedherein by reference in their entireties) are specifically expressed in asubset of taste receptor cells that do not correspond to bitter, sweetor umami sensing cells (FIG. 1). The proteins are localized at theapical tip of taste cells where tastants are detected. PKD1 L3 andPKD2L1 heteromer formation (FIG. 2) is required for functional cellsurface expression and whenever they are expressed in heterologous cellsthey are activated by sour solutions. Therefore, PKD1L3 and PKD2L1function together as sour taste receptors in mammals, although anunderstanding of the mechanism is not necessary to practice the presentinvention and the present invention is not limited to any particularmechanism of action.

In one embodiment, the present invention provides methods for detectingligands, and other modulators, that interact with the sour tastereceptor. In some embodiments, the methods are assays that comprise thePKD1L3 and PKD2L1 proteins, or functional fragments or variants thereof.In some embodiments, the assays comprise human PKD1L3 and PKD2L1proteins, or functional fragments thereof. In some embodiments, thesetwo proteins are co-expressed in tissue culture cells lines, or othercell samples (e.g., gross tissue, tissue explants, primary cells, etc.).In some embodiments, these two proteins are chimeric proteins, whereasone or more of the protein domains is murine in origin while one or moreof the protein domains are of human origin. In some embodiments, testcompounds suspected, or known to be, ligand to sour taste receptors areapplied to the sample and sour taste sensation in the sample issubsequently monitored following application of the test compound. Insome embodiments, the monitoring of the sour taste sensation in a sampleis performed by monitoring calcium influx via fluorescence, although thepresent invention is not limited by the manner in which activity orbinding is monitored. In some embodiments, the ligand inhibits the sourtaste sensation, whereas in other embodiments the ligand enhances thesour taste sensation.

In some embodiments, the PKD1L3 and/or PKD2L1 amino acid sequences arealtered or are provided as part of a chimeric peptide sequence, such aswith an affinity tag to assist with purification, with a localizationtag to assist with intracellular trafficking or localization, and thelike). For example, in some embodiments the sequences of the proteinsare linked, directly or indirectly, (e.g., via a linker) with anaffinity tag (e.g., hemagglutinin A (HA) tag, Rho tag, and the like),for example on the N-terminus of the protein.

In some embodiments, the present invention provides variants orfragments thereof of the wild-type PKD1L3 and/or PKD2L1 gene or geneproduct sequences. For example, a wild-type gene or gene product has thecharacteristics of that gene or gene product when isolated from anaturally occurring source. A wild-type gene is that which is mostfrequently observed in a population and is thus arbitrarily designed the“normal” or “wild-type” form of the gene. In contrast, modified, mutantand variant refer to a gene or gene product that displays modificationsin sequence and or functional properties (i.e., altered characteristics)when compared to the wild-type gene or gene product. It is noted thatnaturally-occurring mutants can be isolated; these are identified by thefact that they have altered characteristics when compared to thewild-type gene or gene product. This is in contrast to synthetic mutantsthat are changes made in a sequence through human (or machine)intervention.

Variants may be generated by post-translational processing of theprotein (e.g., by enzymes present in a producer strain or by means ofenzymes or reagents introduced at any stage of a manufacturing process)or by mutation of the structural gene. Mutations may include sitedeletion, insertion, domain removal and replacement mutations.

Structural and functional equivalents and variants are contemplated withthe present invention. For example, it is contemplated that isolatedreplacement of a leucine with an isoleucine or valine, an aspartate witha glutamate, a threonine with a serine, or a similar replacement of anamino acid with a structurally related amino acid (i.e., conservativemutations) will not have a major effect on the biological activity ofthe resulting molecule. Accordingly, some embodiments of the presentinvention provide variants of sensory receptors, such as sour tastereceptors PKD1L3 and PKD2L1 disclosed herein that contain conservativereplacements. Conservative replacements are those that take place withina family of amino acids that are related in their side chains.Genetically encoded amino acids can be divided into four families: (1)acidic (aspartate, glutamate); (2) basic (lysine, arginine, histidine);(3) nonpolar (alanine, valine, leucine, isoleucine, proline,phenylalanine, methionine, tryptophan); and (4) uncharged polar(glycine, asparagine, glutamine, cysteine, serine, threonine, tyrosine).Phenylalanine, tryptophan, and tyrosine are sometimes classified jointlyas aromatic amino acids. In similar fashion, the amino acid repertoirecan be grouped as (1) acidic (aspartate, glutamate); (2) basic (lysine,arginine, histidine), (3) aliphatic (glycine, alanine, valine, leucine,isoleucine, serine, threonine), with serine and threonine optionally begrouped separately as aliphatic-hydroxyl; (4) aromatic (phenylalanine,tyrosine, tryptophan); (5) amide (asparagine, glutamine); and (6)sulfur-containing (cysteine and methionine) (e.g., Stryer ed.,Biochemistry, pg. 17-21, 2^(nd) ed, WH Freeman and Co., 1981). Whether achange in the amino acid sequence of a peptide results in a functionalhomolog can be readily determined by assessing the ability of thevariant peptide to function in a fashion similar to the referenceprotein. Peptides having more than one replacement can readily be testedin the same manner.

As well, a variant of the present invention includes “nonconservative”changes (e.g., replacement of a glycine with a tryptophan). Analogousminor variations can also include amino acid deletions or insertions, orboth. Guidance in determining which amino acid residues can besubstituted, inserted, or deleted without abolishing biological activitycan be found using computer programs (e.g., LASERGENE software, DNASTARInc., Madison, Wis.).

In some embodiments, the methods and compositions of the presentinvention are combined with compositions and methods of other tastereceptors (e.g., sweet, salty, bitter, umami). Examples of tastereceptor compositions and methods which can be combined or utilized withthe compositions and methods of the present invention include, but arenot limited to, those found in the following United States patent andpatent applications, all of which are incorporated herein by referencein their entireties; U.S. Pat. Nos. 6,955,887, 6,608,176, 20060019346,20050287517, 20050084932, 20040248123, 20040348149, 20040229239,20041219632, 20040209313, 10040209286, 20040191862, 20040175793,20040175792, 20040171042, 20040132134, 20040132075, 20020164645,20020151052, 20020037515. These applications also describe screeningmethods and compound libraries that find use with the present invention.

In one embodiment, the present invention provides methods of identifyingmodulators of the sour taste receptor. A modulator can be a candidate ortest substance that is suspected of modulating (e.g., increasing,decreasing, inhibiting) the activity of the sour taste receptor. As usedherein, the terms “candidate substance” and “test substance” are usedinterchangeably, and each refers to a substance that is suspected tointeract with either PKD1L3, PKD2L1 or the heteromer, including anysynthetic, recombinant, or natural product or composition. A testsubstance suspected to interact with either PKD1 L3 or PKD2L1 or theheteromer can be evaluated for such an interaction using the methodsdisclosed herein. In some embodiments, test substances include, but arenot limited to peptides, oligomers, nucleic acids (e.g., aptamers),small molecules (e.g., chemical compounds), antibodies or fragmentsthereof, nucleic acid-protein fusions, any other affinity agent, andcombinations thereof. A test substance can additionally comprise acarbohydrate, a vitamin or derivative thereof, a hormone, aneurotransmitter, a virus or receptor binding domain thereof, apheromone, a toxin, a growth factor, a platelet activation factor, aneuroactive peptide, or a neurohormone.

In some embodiments, a candidate substance elicits no sour tastesensation. In some embodiments, a candidate substance elicits anincreased, or enhanced, sour taste sensation. In some embodiments, acandidate substance to be tested can be a purified molecule, ahomogenous sample, or a mixture of molecules or compounds. In someembodiments, the test substance is a small molecule. Small molecules maybe comprised in compound libraries of diverse or structurally similarcompounds (e.g, combinatorial chemistry synthesized libraries). In someembodiments, the test substance will include naturally occurring sourcompounds (e.g., derived from plant extracts and the like). Testsubstances can be obtained or prepared as a library. As used herein, theterm “library” means a collection of molecules. A library can contain afew or a large number of different molecules, varying from about tenmolecules to several billion molecules or more. A molecule can comprisea naturally occurring molecule, a recombinant molecule, or a syntheticmolecule. A plurality of test substances in a library can be assayedsimultaneously. Optionally, test substances derived from differentlibraries can be pooled for simultaneous evaluation. Representativelibraries include but are not limited to a peptide library (U.S. Pat.Nos. 6,156,511, 6,107,059, 5,922,545, and 5,223,409), an oligomerlibrary (U.S. Pat. Nos. 5,650,489 and 5,858,670), an aptamer library(U.S. Pat. Nos. 6,180,348 and 5,756,291), a small molecule library (U.S.Pat. Nos. 6,168,912 and 5,738,996), a library of antibodies or antibodyfragments (U.S. Pat. Nos. 6,174,708, 6,057,098, 5,922,254, 5,840,479,5,780,225, 5,702,892, and 5,667,988), a library of nucleic acid-proteinfusions (U.S. Pat. No. 6,214,553), and a library of any other affinityagent that can potentially bind to a T2R76 polypeptide (e.g., U.S. Pat.Nos. 5,948,635, 5,747,334, and 5,498,538). Additionally, a library cancomprise a random collection of molecules. Alternatively, a library cancomprise a collection of molecules having a bias for a particularsequence, structure, or conformation (e.g., U.S. Pat. Nos. 5,264,563 and5,824,483, incorporated herein in their entireties). Methods forpreparing libraries containing diverse populations of various types ofmolecules are known in the art, for example as described in U.S. patentscited herein above. Numerous libraries are also commercially available.

In some embodiments, ligands that inhibit sour taste sensation are usedin the pharmaceutical industry to create more palatable medicaments. Insome embodiments, ligands that inhibit sour taste sensation are suitablefor oral administration and may be presented as adjuvants in capsules,cachets or tablets, wherein the medicament preferably contains apredetermined amount of ligand sufficient to inhibit the sour tastesensation.

In some embodiments, ligands that inhibit sour taste sensation are usedwith food products and beverages. In some embodiments, ligands thatinhibit sour taste sensation are added to, applied to, or applied on,food products that impart a sour taste sensation (e.g., for example,broccoli and green grapes). In some embodiments, ligands that inhibitsour taste sensation are added to beverages (e.g., for example,grapefruit juice, lime juice and lemon juice).

In one embodiment, the present invention relates to compositions andmethods relating to RNA inhibition of the sour taste receptor. In someembodiments, the translation of either PKD1 L3 or PKD2L1 is inhibited byapplication of a short interfering siRNA (siRNA). In some embodiments,the siRNA targets the expression of one or both of the murine sour tastereceptor proteins. In some embodiments, the siRNA targets the expressionof one or both of the human sour taste receptor proteins.

In one embodiment, the present invention relates to compositions andmethods for inhibition of the sour taste receptor by using an antibodyto either PKD1L3 or PKD2L1, or both, or fragments thereof. In someembodiments, antibodies are administered with pharmaceutical medicamentsand treatments. In some embodiments, the antibodies are co-administeredwith food stuffs (e.g., broccoli, cauliflower, spinach, etc.) thattrigger sour taste receptors.

In one embodiment, the present invention relates to compositions andmethods for inhibition of the sour taste receptor by using a smallmolecule to PKD1 L3, PKD2L1, or both, or fragments thereof. In someembodiments, the small molecules are administered with pharmaceuticalmedicaments and treatments. In some embodiments, the small molecules areco-administered with food stuffs (e.g., broccoli, cauliflower, spinach,etc.) that trigger sour taste receptors.

In one embodiment, the methods of the present invention are used todefine ligands that enhance sour taste sensation. In some embodiments,ligands that enhance sour taste sensation are added to human consumableproducts, such as candy, gummy worms, powdered candy, chewing gum,libations and elixirs.

In one embodiment, PKD1L3 and PKD2L1 can be used to created transgenicanimals (e.g., mice, rats, hamsters, guinea pigs, ungulates, zebrafish,pigs, birds, etc.). In some embodiments, the transgenic animals arecreated such that the sour taste receptor is overexpressed. In someembodiments, the transgenic animals are created such that sour tastereceptor expression is knocked out (e.g., does not express thereceptor). In some embodiments, the transgenic animal has one of PKD1L3or PKD2L1 genes knocked out. In other embodiments, the transgenic animalhas both PKD1L3 and PKD2L1 genes knocked out. In some embodiments, thetransgenic animal expresses one or both of human PKD1 L3 and PKD2L1. Insome embodiments, the transgenic animals express a chimeric protein forPKD1L3, PKD2L1 or both. Techniques for the preparation of transgenicanimals are known in the art. Exemplary techniques are described in U.S.Pat. No. 5,489,742 (transgenic rats); U.S. Pat. Nos. 4,736,866,5,550,316, 5,614,396, 5,625,125 and 5,648,061 (transgenic mice); U.S.Pat. No. 5,573,933 (transgenic pigs); U.S. Pat. No. 5,162,215(transgenic avian species) and U.S. Pat. No. 5,741,957 (transgenicbovine species), all patents being incorporated herein by reference intheir entireties.

In one embodiment, computer modeling and searching technologies are usedto identify compounds, or improvements of already identified compounds,that can modulate the sour taste receptor expression or activity. Havingidentified such a compound or composition, the active sites or regionsare identified. Such active sites might typically be ligand bindingsites, such as the interaction domains of a portion of a ligand with thesour taste receptor itself (e.g., either PKD1L3 or PKD2L1 alone, or theheteromer), or the interaction domains of a ligand with the wild-typesour taste receptor in comparison to the interaction domains of ligandwith a mutant (e.g., change in the nucleic acid or amino acid sequence,or deletions, insertions, truncations of a gene or protein) sour tastereceptor. In some embodiments, the active site can be identified usingmethods known in the art including, for example, from the amino acidsequences of peptides, from the nucleotide sequences of nucleic acids,or from study of complexes of the relevant compound or composition withits natural ligand. In some embodiments, chemical or X-raycrystallographic methods can be used to find the active site by findingwhere on the heteromer the complexed ligand is found. In someembodiments, the three dimensional geometric structure of the activesite is determined (e.g., by known methods including X-raycrystallography). In further embodiments, solid or liquid phase nuclearmagnetic resonance can be used to determine certain intra-moleculardistances. In some embodiments, partial or complete geometric structuresof the heteromer alone, or with ligand interaction, is accomplished byhigh resolution electron microscopy. For example, the geometricstructures can be measured with a complexed ligand, natural orartificial, thereby increasing the accuracy of the active sitestructure. In another embodiment, the structure of the wild-type sourtaste receptor is compared to that of a mutant sour taste receptor. Insome embodiments, rather than solve the entire structure, the structureis solved for the protein domains that are changed between the wild typeand mutant sour taste receptor.

In one embodiment, the present invention provides cells expressing wildtype or chimeric PKD1L3 and/or PKD2L1 proteins. In some embodiments, thecells are human cells. In some embodiments, the human cells are humanembryonic kidney 293T cells. In some embodiments, the cells are murinein origin. In some embodiments, the wild type proteins are murine inorigin, whereas in other embodiments the wild type proteins are human inorigin. In some embodiments, the chimeric protein contains domains,regions, or fragments of both human and murine PKD1L3 and/or PKD2L1proteins. In some embodiments, the chimeric proteins express domains,regions, or fragments of human and/or murine PKD1L3 and/or PKD2L1 inconjunction with non-human or non-murine homologous protein domains(e.g., Xenopus, zebrafish, C. elegans, for example). In someembodiments, the cells comprise a chimeric PKD1L3 and/or PKD2L1 proteinsare used to study structure/function relationships, and other assays tocharacterize sour taste receptor activity and function.

In some embodiments, derivatives of human embryonic kidney 293T cellsare used for optimal expression of human PKD1 L3 and/or PKD2L1 proteinsor fragments thereof at the cell membrane. In some embodiments, aderived human embryonic kidney 293T cell line is a Hana3A cell lineconfigured to express, via stable or transient transfection, one or moreof receptor transporting proteins (e.g., RTP1, RTP2), receptorexpressing enhancer proteins (e.g., REEP1) (Behrens et al., 2006, J.Biol. Chem. 281:20650-20659; incorporated herein by reference in itsentirety), and/or the olfactory neuron specific G-protein G_(olf)protein (Jones & Reed, 1989, Science 244:790-795, incorporated herein byreference in its entirety). In some embodiments, a further derivation ofthe Hana3A cell line is the “44” cell line configured to express, viastable or transient transfection, one or more of brain synembrin(Ric8B), the heat shock protein 70 (HSP70) homolog HSC70T, and/or anRTP1A1 protein. For example, expression of human PKD1L3 and human PKD2L1in the cell membrane of 44 cells expressing one or more of Ric8B, HSC70Tand/or RTP1A1 is enhanced as compared to expression is Hana3A cells or293T cells.

EXAMPLES Example 1 In Situ Hybridizations

Procedures for non-radioactive hybridization were previously described(Saito et al., 2004, Cell 119:679-691). Briefly, digitonin (Dig) labeledRNA probes were hybridized, washed and detected by alkaline phosphataseconjugated anti-Dig antibodies followed by incubation with NBT/BCIP. Fortwo-color fluorescent in situ hybridization, RNA probes were labeledwith Dig or FITC (Roche). FITC labeled probes were detected by horseradish peroxidase (HRP) conjugated anti-FITC antibodies followed byTSA-Cy3 (Perkin-Elmer). HRP was inactivated by incubating with PBScontaining 1% hydrogen peroxide for 30 min., and Dig labeled probes weredetected by HRP conjugated anti-Dig followed by TSA-FITC.

Example 2 Immunoprecipitation

Protocols used for immunoprecipitation were previously described inSaito et al, 2004.

Example 3 Cell Surface Protein Expression

Protocols used for cell surface expression of proteins were previouslydescribed in Saito et al., 2004.

Example 4 Cell Culture, Gene Cloning and Calcium Imaging

Cell tissue culture was performed as previously described in Saito etal., 2004. The PKD1L3 gene (SEQ ID NO:1) was cloned into the mammalianexpression vector pDisplay (Invitrogen), and the PKD2L1 gene (SEQ IDNO:5) was cloned into the mammalian expression vector pCI (Promega). Forcalcium imaging, pDisplay-PKD1L3 and/or pCI-PKD2L1 were transfected intocells (previously seeded on glass coverslips) using Lipofectamine 2000(Invitrogen). Following incubation, the transfected cells were loadedwith fluo-4 (Molecular Probes) and fura-red (Molecular Probes) for 45min. at room temperature prior to analysis.

Results

The mouse genome contains at least 33 TRP channel members. To identifyTRP ion channel members functioning in taste transduction, in suithybidizations were performed using probes against all 33 TRP channelmembers (Corey et al., 2004, Nature 432:723-730) against sections ofcircumvallate papilla of the mouse taste tissue. Probes for TRPM5labeled a subset of taste cells, and probes for PKD1L3 and PKD2L1 alsohybridized to taste cells. A similar expression pattern was observedwith rat circumvallate papilla. Other TRP channels did not show robustexpression in taste cells.

In circumvallate papilla, around 20% of the taste cells expressed PKD1L3and PKD2L1. To examine the expression of PKD1 L3 and PKD2L1 in differenttaste areas, in situ hybridization with sections from circumvallate,foliate and fungiform papilla, and palate was performed. PKD2L1expression was observed in a subset of taste cells in all four differenttaste areas, whereas PKD1 L3 expression was only seen in circumvallateand foliate papillae. Additional in situ hybridization experiments didnot reveal significant expression of other PKD family members infungiform papilla or palate.

To investigate the correlation of TRPM5, PKD1L3 and PKD2L1 expressioncells in taste buds, double-labeled fluorescent in situ hybridizationswere performed. In circumvallate and foliate papilla, almost all of thePKD1L3 positive cells were also PKD2L1 positive, indicating these twomolecules are expressed in the same cells. In contrast, TRPM5 signalsdid not co-localize with PKD2L1 or PKD1L3 indicating different tastecells express TRPM5 and PKD1L3/PKD2L1. In fungiform papilla and palate,PKD2L1 positive cells were PKD1L3 negative, confirming the absence ofPKD1L3 expression in these two areas.

To examine mRNA expression of PKD and PKD2L1 in different tissues,RT-PCR was performed using mRNA for 16 different tissues including tastetissues (circumvallate and foliate papillae). Both PKD and PKD2L1 wereabundantly expressed only in taste tissues and testis, whereas they wereabsent or only faintly expressed in all other tissues tested (GAPDHpositive control RT-PCR showed expression in all tissues).

Taste reception occurs at the taste pore where the apical tip of eachtaste cell dendrite topped with microvilli is accumulated. Todemonstrate the co-localization of PKD1L3 and PKD2L1 at the apical tipof the taste cell dendrite, antibodies against PKD2L1 were generated toanalyze the PKD2L1 cellular localization within the taste cells.Immunostaining with rat and mouse circumvallate and foliate tastetissues demonstrate that PKD2L1 localized at the apical end of a subsetof taste cells at the taste pore area, with weaker labeling throughoutthe positive cells. Preincubation of the antibody with peptide antigen(10 ng/ml) abolished the taste cell staining, thereby confirming thespecificity of the antibody. Monoclonal IP3R-3 antibody marks PLCb2 andTRPM5 expressing bitter, sweet and umami sensing cells (Clapp et al.,2001; Miyoshi et al., 2001). Double staining using antibodies againstPKD2L1 and IP3R-3 revealed different sets of taste cells were expressingPKD2L1 and IP3R-3, consistent with mRNA expression analysis. Therefore,the interaction between PKD1L3 and PKD2L1 is consistent with the rolefor PKD1L3 and PKD2L1 in taste reception.

Since Hanaoka et al. (2000) had previously suggested that the C-terminalcytoplasmic domains of related PKD1 and PKD2 domains interacted andcreated functional channel expression, experiments were performed toinvestigate whether PKD1L3 and PKD2L1 also formed functional heteromericreceptors. Cell surface expression of PKD1 L3 was investigated with andwithout the presence of PKD2L1. PKD1L3 was tagged with HA at theN-terminal extracellular domain. When PKD1L3 was expressed alone inHEK293T cells, no cell surface expression was observed when compared tocontrol BFP signals (PKD1 L3 was observed when the cells werepermeabilized and stained demonstrating cytoplamic expression). It hadbeen previously demonstrated by Murakami et al. (2005, J. Biol. Chem.280:5626-5635) that PKD2L1 alone is not transported to the cell surfacein heterologous cells. Therefore, interaction between the two moleculesis necessary for their cell surface expression.

Bitter taste receptors (T2R5) and sweet and umami receptors (T1R5) areco-expressed with TRPM5, PLCb2 and IP3R-3 proteins. Since PKD1L3 andPKD2L1 positive cells do not co-localize with TRPM5 or IP3R-3 positivecells, it was tested whether these two proteins were involved in anothertaste sensation; such as sour or salty. To examine whether PKD1L3/PKD2L1 function as taste receptors, calcium imaging experiments werecarried out using HEK293T cells transiently expressing PKD1L3 and/orPKD2L1. The cells were transfected with expression vectors encoding PKD1L3 and/or PKD2L1, loaded with calcium sensitive dyes (fluo-4 andfura-red), and stimulated with various taste chemicals and osmolaritysolutions. When calcium concentration inside the cell is upregulatedupon stimulation with ligands, the fluo-4 signal increases whereas thefura-red signal decreases, thereby allowing ratiometric measurements ofintracellular calcium concentration (Wong et al., 2002, Nat. Neurosci.5:1302-1308). It was demonstrated that cells expressing both PKD1L3 andPKD2L1 responded to solutions containing citric acid (25 mM, pH 2.6),whereas cells expressing either PKD1L3 or PKD2L1, or neither of them,showed little or no calcium response when treated with citric acid. Whenextracellular calcium ions were eliminated from the bath solution, thecalcium response from citric acid was abolished, demonstrating thatcalcium ionw were coming from the extracellular solution. Theexperiments demonstrate that PKD1 L3 and PKD2L1 form functional channelsthat are activated by citric acid. Citric acid elicits a more sourresponse at the same pH when compared to hydrochloric acid. Consistentto this notion, hydrochloric acid at the same pH caused much less of acalcium response in cells expressing both proteins. Further, function ofthe two protein heteromer was not inhibited by the ASIC inhibitoramiloride or the HCN inhibitor Cs+. Additionally, PKD1L3/PKD2L1 did notrespond to salt (NaCl), bitter chemicals (quinine, cyclohexamide, PROP),sucrose, saccharin, or the umami compounds I-glutamate and IMP.Therefore, heteromers of PKD1L3 and PKD2L1 function as sour tastereceptors.

PKD1L3 has homology to PKD1; both have a large extracellular domainfollowed by eleven transmembrane domains, whereas PKD2L1 is similar toPKD2; both have six transmembrane domains like most of the TRP channelmembers. PKD1 does not appear to function as an ion-conducting channel,but rather plays a role in sensing mechanical flow, whereas PKD2 forms afunctional ion-conducting channel (Gonzalez-Perrett et al., 2001, Proc.Natl. Acad. Sci. 98:1182-1187; Nauli et al., 2003, Nat. Genet.33:129-137). Chen et al. (1999) showed that PKD2L1 was capable offorming a functional calcium permeable channel, whereas it was not knownwhether PKD1 L3 alone could form an ion-conducting channel. Calciumimaging experiments found that acid stimulation (e.g., citric,hydrochloric, maleic) opens calcium permeable channels. The presentinvention is not limited to a particular mechanism. Indeed, anunderstanding of the mechanism is not necessary to practice the presentinvention. Nonetheless, it is contemplated that PKD1L3 functions as asour sensing receptor and PKD2L1 functions as an ion-conducting channel.An additional possibility is contemplated, in that PKD2L1 functions as asour receptor and PKD3L1 functions as a facilitator of PKD2L1expression.

Sour sensation is not a simple measurement of pH in a solution. Forexample, at the same pH, citric acid or acetic acid tastes more sourthan hydrochloric acid (Ganzevles and Kroeze, 1987, Physiol. Behay.40:641-646; Makhlouf and Bum, 1972, Gastroenterology 63:67-75).Similarly, calcium imaging experiments using mouse taste tissue slicesshowed that citric acid is a more potent sour ligand than hydrochloricacid at the same pH (Richter et al., 2003). The present invention is notlimited to a particular mechanism. Indeed, an understanding of themechanism is not necessary to practice the present invention.Nonetheless, it is contemplated that sour taste receptors do notfunction as mere acid pH sensors. The experiments presented hereindemonstrate that citric acid is more potent than hydrochloric acid inactivating PKD1 L3/PKD2L1 heteromers at the same pH. It is contemplatedthat citrate ions or an undissolved form of citric acid interacts withPKD1 L3 and/or PKD2L1 and enhances the sensitivity of the hydrogenactivated receptor. A similar mechanism can be found in umami tastesensations, where some nucleotides such as IMP potentiate the activationof the umami receptor T1R1/T1R3 to 1-amino acids (Li et al., 2002;Nelson et al., 2002).

It is not well understood why both PKD1L3 and PKD2L1 are needed for cellsurface expression. The present invention is not limited to a particularmechanism. Indeed, an understanding of the mechanism is not necessary topractice the present invention. Nonetheless, it is contemplated that asthe C-terminal cytoplasmic domain of PKD2L1 contains endoplasmicreticulum (ER) retention signals (Murakami et al., 2005), the C-terminalcytoplasmic domain of PKD1L3 also contains ER retention signals and theinteractions between PKD2L1 and PKD1L3 mask these signals, therebyallowing the complex to be transported to the cell surface.

Previous studies have shown that different taste cells are responsiblefor sensing bitter, sweet or umami taste. It is demonstrated herein thatPKD1L3/PKD2L1 expressing cells are segregated from TRPM5 and IP3R-3expressing bitter, sweet or umami taste cells, thereby demonstratingthat a subset of cells are sour sensing cells. Additionally, Caicedo etal. (2002, J. Physiol. 544:501-509; Richter et al., 2003) have shownthat 23-25% of taste cells are activated by citric acid with calciumimaging of taste bud slices. This correlated with the present findingsthat approximately 20% of taste cells express PKD1L3 and PKD2L1.

All publications and patents mentioned in the present application areherein incorporated by reference. Various modification and variation ofthe described methods and compositions of the invention will be apparentto those skilled in the art without departing from the scope and spiritof the invention. Although the invention has been described inconnection with specific preferred embodiments, it should be understoodthat the invention as claimed should not be unduly limited to suchspecific embodiments. Indeed, various modifications of the describedmodes for carrying out the invention that are obvious to those skilledin the relevant fields are intended to be within the scope of thefollowing claims.

1. A method for identifying a sour taste receptor ligand, comprising: a)providing i) a sample comprising a sour taste receptor wherein saidfunctional sour-taste receptor comprises two or more polycystic kidneydisease (PKD) proteins, wherein said two or more PKD proteins includePKD1 L3 and PKD2L1, and ii) a test compound; b) exposing said testcompound to said sample; and c) measuring the activity of said sourtaste receptor in said sample in response to said test compound.
 2. Themethod of claim 1, wherein said sample is a cell line.
 3. The method ofclaim 1, wherein said cell line is a 293T cell line.
 4. The method ofclaim 1, wherein said PKD1L3 and PKD2L1 are either human or murine. 5.The method of claim 1, wherein said test compound is from a listconsisting of a naturally occurring molecule, a synthetically derivedmolecule, or a recombinantly derived molecule.
 6. The method of claim 1,further comprising a reporting agent.
 7. The method of claim 6, furthercomprising the step of d) detecting the presence or absence of a sourtaste receptor ligand based upon said reporting agent activity.
 8. Themethod of claim 6, wherein said reporting agent is a fluorophore.
 9. Themethod of claim 8, wherein said fluorophore is from a group consistingof fluo-4 and fura-red.